On the modulation of innate immunity by plant-parasitic cyst nematodes

Plant-parasitic cyst nematodes are major agricultural pests worldwide. These obligate endoparasites invade the roots of host plants where they transform cells near the vascular cylinder into a permanent feeding site. Plants possess a multilayered innate immune system consisting of different types of extracellular and intracellular immune receptors. These enable detection of most invading nematodes and initiate immune responses that result in resistance. Many plant pathogens use effectors to overcome resistance. Here, modulation of plant innate immunity by plant-parasitic cyst nematodes was investigated. Extracellular immune receptor signaling and hormone-mediated signaling pathways were found to contain infection of susceptible Arabidopsis thalianawith Heterodera schachtii. A large family of effectors was identified in Globodera rostochiensis. One of these so-called SPRYSECs interacted with a novel CC-NB-LRR type resistance protein of a susceptible tomato without inducing resistance responses. Instead, the effector was found to suppress defense-related programmed cell death and resistance mediated by several CC-NB-LRR type resistance proteins. In addition, a secreted antimicrobial peptide was identified in G. rostochiensis. Plant-parasitic cyst nematodes thus most likely secrete effectors that protect against plant immune responses and secondary infections. The current evidence for the existence of immune modulating effectors is reviewed and directions for further research are given.